The light curve of the companion to PSR B1957+20

TL;DR

This study analyzes the light curve of the secondary star in the PSR B1957+20 system using combined data, modeling the system's inclination and properties of the companion star.

Contribution

It provides new multi-band light curve data and constrains the system inclination and companion star characteristics using the ELC modeling approach.

Findings

System inclination constrained to 65 +/- 2 degrees.

Secondary star is not filling its Roche lobe.

Temperature gradient across the star is physically sustainable.

Abstract

We present a new analysis of the light curve for the secondary star in the eclipsing binary millisecond pulsar system PSR B1957+20. Combining previous data and new data points at minimum from the Hubble Space Telescope, we have 100% coverage in the R-band. We also have a number of new K_s-band data points, which we use to constrain the infrared magnitude of the system. We model this with the Eclipsing Light Curve code (ELC). From the modelling with the ELC code we obtain colour information about the secondary at minimum light in BVRI and K. For our best fit model we are able to constrain the system inclination to 65 +/- 2 degrees for pulsar masses ranging from 1.3 -- 1.9 M_sun. The pulsar mass is unconstrained. We also find that the secondary star is not filling its Roche lobe. The temperature of the un-irradiated side of the companion is in agreement with previous estimates and we find that the observed temperature gradient across the secondary star is physically sustainable.